The objective of this project is to obtain knowledge concerning the functional organization of mammalian neuroendocrine cells in the supraoptic nucleus (NSO) of the unanesthetized sheep. Mature sheep are surgically prepared under metofane anesthesia with a cranial platform-cylinder arrangement and pituitary stimulating electrodes in preparation for subsequent microelectrode recordings from single, antidromically identified supraoptic units. Spontaneous firing patterns (silent, continuously active, and low frequency bursting) are characterized in the unanesthetized animal and their response observed to vaginal distention and to combinations of carefully quantitated changes in plasma osmolality and right atrial pressure. It will be determined if the different neuroendocrine cell discharge patterns are a reflection of certain sensory input connections and associated with a specific hormone secretion, and/or if they are a function of varying recruitment thresholds to stimulus intensity. The convergency of sensory input, distribution of sensotry specific neuroendocrine cells, role of cell size (indicated by axon conduction velocity) in osmotic thresholds, and the dynamic characteristics of the osmotic and volume receptor-neuroendocrine cell system will be determined.